The present invention relates to a polarization converter which efficiently converts a randomly polarized light (unpolarized light) into a linearly polarized light.
Recently, large screen image projectors utilizing a light modulators of twisted nematic type liquid crystals are becoming popular. For this type of image projector, a linearly polarized light is needed as a projection light, and this is true of other types of image projectors utilizing optical devices having an electrooptic effect for the operation of the projectors.
In this regard, a conventional system to obtain such a linearly polarized light is that a randomly polarized light generated from a high intensity light source such as an incandescent or arc lamp, is passed through an optical analyzer. However, this conventional system is poor in efficiency as the energy of the utilizable linearly polarized light is less than half of the energy of the original randomly polarized light.
Some improvements with respect to this efficiency problem are disclosed in the following prior art:
(1) Polarization Converter Element for High Intensity Liquid Crystal Projection System, preprint No. 5-page 34 for the 1989 Fall Convention of The Institute of Electronics, Information and Communication Engineers, in which a P-polarized light is converted into an S-polarized light or vice versa by a total reflection system. (Note: P-polarized light is a linearly polarized light of which the plane of polarization is perpendicular to the plane of incidence of the light before conversion, whereas an S-polarized light is a linearly polarized light of which the plane of polarization is perpendicular to the P-polarized light.) PA0 (2) Japanese Laid-open Patent Application No. 1 (1989)-201693 in which a P-polarized light and an S-polarized light are combined together after being modulated by respective liquid crystal modulators.
However, the system of prior art (1) involves many optical components that make the system too complicated and the P-polarized light and the S-polarized light produced in the system have their respective light paths differing from each other and this is liable to cause disagreement between the two lights upon landing on the light modulator (liquid crystal panel) for image projection. The system of the prior art (2) requires two independent light modulators (liquid crystal panels) to produce modulated light beams to be combined thereafter by a polarization beam splitter, and because of this optical arrangement, the system is liable to cause disagreement between two images respectively produced by the two independent light modulators.